Microstructural, mechanical, and corrosion properties of plasma-nitrided CoCrFeMnNi high-entropy alloys

Plasma nitriding is used for materials that are challenging to nitride by conventional techniques. Because of the sputtering effect of positive ions during glow discharge, protective oxide films on the surfaces of materials including stainless steels, aluminum alloys, and titanium alloys, can be effectively removed by ion and energetic particle bombardment; thus, nitrogen mass transfer from the plasma into the component subsurface can be achieved effectively. Plasma nitriding therefore has potential applicability to treat high-entropy alloys (HEAs) containing large amounts of strong oxide-forming elements such as chromium. In this study, the effects of plasma nitriding were investigated on the microstructural, mechanical, and corrosion properties of a CoCrFeMnNi HEA with an fcc structure that is soft and ductile. The HEA was produced by melting pure metals and casting. Direct-current plasma nitriding was then performed in a gas mixture of 25% N-2 and 75% H-2 for 54 ks at 673-823 K under 200 Pa with an auxiliary cathodic screen. After nitriding, the nitrided samples were examined using scanning electron microscopy, X-ray diffraction (XRD), Vickers microhardness testing, electron probe micro-analysis, and glow discharge optical emission spectroscopy (GD-OES). After nitriding, GD-OES revealed that the thickness of the nitriding layer tended to increase with increasing nitriding temperature. XRD analysis revealed that an fcc supersaturated solid solution phase was formed on surfaces nitrided below 723 K, whereas a CrN phase was formed on those nitrided above 723 K. The Vickers microhardness of the nitrided sample surfaces reached approximately 1300 HV. Ball-on-disk wear tests revealed that the wear loss of nitrided samples was considerably lower than that for untreated samples. Finally, the pitting corrosion resistance of samples nitrided at 673 K was higher than that of untreated samples.